The present invention relates generally to printing blankets, and in particular to a method of securing a fabric web to a travelling printing blanket without having to resort to adhesives, and to needles suitable for this purpose.
Printing blankets are in effect endless belts on which a web is supported when it is to be printed in a printing machine. Frequently, such printing machines have two or more printing stations at which different patterns or colors are applied to the web in successive operations. To assure that from one station to the next the web will not move out of registry it is necessary that the web be prevented from shifting with reference to the printing blanket on which it is supported. This is particularly true if the web is of the type that is inherently extensible, for instance a fabric, such as a high-nap fabric such as rugs, carpets and the like.
According to the prior art this fixed connection with the printing blanket is achieved by bonding the web to the surface of the printing blanket with an adhesive that permits the web later to be stripped off again. However, with certain types of fabric webs this creates difficulties. Particularly in the case of high-nap fabric webs, such as carpets, rugs or the like, both the top side and the bottom side of the fabric web are quite rough and it is therefore not possible to bond the bottom side of the web to the printing blanket over their entire interface. Instead, a bonding usually takes place only at a series of spacedapart locations which does not afford the desired effect. Moreover, although the bonding agent or adhesive must permit the material to be subsequently stripped off the printing blanket again, it must nevertheless have excellent bonding ability and in the case of high-nap fabric webs this usually means that when the web is subsequently stripped off the printing blanket, a quite large number of nap fibers or threads are lost from the web, because they are simply pulled out of the web when the latter is stripped off, due to the fact that they adhere too tightly to the bonding agent. Added to these disadvantages is the fact that once the fabric web is stripped off the printing blanket, the latter must be washed to remove all traces of the adhesive bonding agent, which is frequently difficult. The adhesive itself is not inexpensive.
These problems are not new. They have been recognized in the prior art and an attempt was made to overcome them by using needles to hold the fabbic web in place. However, heretofore these needles were always secured in separate relatively narrow strips or bands which were then in turn secured to the printing blanket. The prior art has never attempted to provide needles directly in the printing blanket itself, with good reason. The separate bands which are relatively narrow and have a width of only approximately 50 - 60 millimeters, could be readily produced in uniform thickness, even if the thickness was relatively substantial. A relatively substantial thickness, however, was necessary in order to assure that the completely cylindrical shafts of the needles were properly retained in the material of the bands in order to prevent them from lying down or extending skew to the exposed surface of the bands rather than normal thereto as is intended. The inclusion of the needles directly in the printing blankets was held to be impossible, especially in the case of printing blankets which are used for rotary printing and which may have a width of up to 5 meters or possibly even more. Because of the various requirements which are made of such printing blankets and are known to those skilled in the art, these printing blankets must be relatively thin, having a thickness on the order of approximately 2 - 3 millimeters. A few experimental attempts carried out in the industry and aimed at providing needles directly in the printing blankets, met with complete failure because the thickness of the printing blankets was simply insufficient to properly maintain the needles in such a position that their projecting tips would extend normal to the plane of the printing blanket surface. The result was that in use the needles would extend skew in one or the other direction and permit shifting of the fabric web (rather than preventing it) when forces acted upon the web tending to shift it lengthwise of the printing blanket.